In proportion as the valve lifts and the spring compresses, its resistance increases, and this tends to impair the accuracy of the valve. This, however, is offset from the fact that when the valve rises from its seat it presents a greater area for the steam to act against.
The area of safety valve required by the English Board of Trade is about 1⁄2 square inch of valve area per square foot of fire grate area.[67]
[67] See [page 409], Vol. II., for safety valve calculations.
There are three test cocks, which are sometimes placed in a diagonal row on the front of the boiler, and sometimes on the fitting for the gauge glass. The top test cock shows highest level to which the water should rise in the boiler, and the lowest one the lowest level, the middle cock indicating the average. There is usually a vertical distance of about 4 inches between the test cocks, which gives a permissible range of 8 inches in the level of the water in the boiler.
Test cocks are prevented from choking with scale by passing a wire through the cock and clear into the boiler, a plug being provided, which, when removed by unscrewing, permits the insertion of the wire.
This cleaning must obviously be performed when there is no steam on the boiler.
A gauge glass is a glass tube whose bore is open to the boiler. It is fitted at each end to a brass socket that is screwed into the boiler, each socket having a cock that permits communication between the gauge glass and the boiler to be shut off in case the glass should break. The bottom socket is also fitted with a cock, which, on being opened, permits the water and steam to blow through the gauge glass and clean it of scum or dirt.
The gauge glass must be plainly in sight, and placed at such a height that when the desired quantity of water is in the boiler it will half fill the gauge glass.
Glass water gauges, instead of attaching to the boiler, are sometimes fitted to a fitting that connects to the top and bottom of the boiler, with the object of attaining, for the gauge glass, water free from the scum and impurities which collect at and near the surface of the water in the boiler. This fitting should have cocks in each pipe leading to the boiler, so that in case the gauge glass breaks, steam can be shut off from the boiler.
In some cases the test cocks are also attached to this fitting, and in this case the construction should be such that shutting off communication between the gauge glass and the boiler will not at the same time shut off communication between the test cocks and the boiler.